1. Field of the Invention
The present invention relates to a transparent, conductive film forming method of forming a transparent, conductive film on a semiconductor layer stacked on a substrate, by sputtering, a semiconductor-layer defective region compensation method of compensating a defective region produced in a semiconductor layer stacked on a substrate, a photovoltaic element in which a transparent, conductive film is formed by sputtering, on a semiconductor layer stacked on a substrate, and a method of producing a photovoltaic element.
2. Related Background Art
In recent years, various studies have been and are being conducted toward practical use of photovoltaic power generation by solar cells. In order to cover some of power demands by the solar cells, they need to meet the requirements of sufficiently high photoelectric conversion efficiency of solar cells used, excellent reliability, and capability of volume production.
First, common sputtering techniques will be described below.
In general, as methods of depositing a transparent, conductive film on a substrate by sputtering, there are two types of methods proposed: one is a method of effecting sputtering in Ar gas, using an oxide of In2O3xe2x80x94SnO2 or the like as a target; the other is a reactive sputtering method of sputtering an alloy of Inxe2x80x94Sn or the like in mixed gas of Ar and O2. The former permits formation of a film with low electrical resistance and high transmittance immediately after sputtering, but involves the difficulty of increasing film forming rates.
On the other hand, the latter reactive sputtering method allows increase of film forming rates. Particularly, in the case of DC magnetron sputtering apparatus using a cylindrical, rotatable target, as described in U.S. Pat. Nos. 4,356,073 and 4,422,916, it is reported that the utilization efficiency of the target material is approximately 2.5 to 3 times higher than those of the general planar type (Kinouzairyou (Functional materials), Vol. 11, No. 3, pp. 35-41, March 1991). The advantages of this reactive sputtering method include saving of the target material and great decrease of production stop time for exchange of targets. Accordingly, the DC magnetron sputtering apparatus using the rotatable target is suitable for volume production.
This reactive sputtering, however, requires extremely narrow adequate ranges of film formation conditions, particularly, flow rates of gas; for example, where a transparent, conductive film was formed on a sheet-like substrate of a large area, it was difficult to control the film formation parameters such as evenness of sheet resistance and transmittance, discharge stability, and so on.
The reactive sputtering method employing a plasma emission monitor (hereinafter referred to as xe2x80x9cPEMxe2x80x9d) is known as a method overcoming the disadvantage.
Reference should be made to S. Schiller, U. Heisig, Chr. Korndorfer, J. Strumpfel, V. Kirchhoff xe2x80x9cProgress in the Application of the Plasma Emission Monitor in Web Coatingxe2x80x9d (Proceedings of the 2nd International Conference on Vacuum Web Coating, Fort Lauderdale, Fla., USA, October 1988).
This PEM is a device for collecting plasma emission by a collimator, guiding the emitted light through a spectroscope to a photomultiplier tube (photomultiplier), photoelectrically converting the light into an electric signal, and monitoring the state of the plasma, based on the electric signal. The device has a function of setting the sensitivity of the photomultiplier of the PEM at a certain value and regulating the flow rate of introduction of reactive gas so as to keep the emission intensity of the plasma constant.
Japanese Patent Application Laid-Open No. 11-29863 discloses the technique of forming a film of ITO (Indium Tin Oxide) on a substrate. This technique is generally a method of setting a substrate in a film forming chamber, inducing discharge in the film forming chamber in a state in which sputter gas is introduced and reactive gas is not introduced thereinto, adjusting the sensitivity of the device for monitoring the emission intensity of the plasma such that the emission intensity of the plasma of the discharge becomes a predetermined value, and sputtering the target while controlling the introducing amount of the reactive gas so as to keep the film forming rate constant. Namely, it is the technique of forming a uniform film by regulating the flow rate of introduction of the reactive gas (O2) so as to keep the plasma emission intensity of In (at the wavelength=451.1 nm) constant during formation of the ITO film.
These techniques made it feasible to produce a satisfactorily good deposited film on a satisfactorily stable basis in the reactive sputtering methods.
The common defect removing techniques will be described below.
The amorphous silicon (hereinafter referred to as xe2x80x9ca-Sixe2x80x9d) solar cells are drawing attention, because they can be produced at lower cost and have higher mass producibility than the solar cells produced using crystalline Si and others. The reason for it is that it is possible to use readily available gas such as silane or the like as source gas, decompose it by glow discharge, and form a deposited film of a semiconductor film or the like on a relatively inexpensive, belt-like substrate such as a metal sheet, a resin sheet, or the like.
Incidentally, the output power of about 3 kW is necessary for applying the solar cells to power supply at ordinary households. With use of the solar cells having the photoelectric conversion efficiency of 10%, the area in that case is 30 m2, and it is thus necessary to prepare the solar cells of large area. It is, however, very hard to produce the solar cells without defects over a large area because of the production steps of solar cells.
For example, it is known that low-resistant portions appear at grain boundary regions in polycrystalline solar cells and that in the thin film solar cells such as those of a-Si, defects are produced by influence of dust or the like during formation of semiconductor layers to become the cause of shunts and decrease the photoelectric conversion efficiency and yield significantly.
Further, causes of production of defects and their effect include the following; for example, in the case of the a-Si solar cell deposited on a stainless steel substrate, the substrate surface cannot be regarded as a perfectly smooth surface, but has flaws and dents, a back surface reflecting layer of uneven structure is provided on the substrate for the purpose of effective use of incident light, it is thus difficult for thin film semiconductor layers several ten nm thick such as n- or p-layers to completely cover such a surface, defects are produced by dust or the like during film formation, and so on.
Where the semiconductor layers between the lower electrode and the upper electrode of the solar cell are lost because of the defects or the like to cause direct contact between the lower electrode and the upper electrode or where the semiconductor layers are not lost completely but themselves have a low resistance to cause shunts between the upper electrode and the lower electrode, the electric current generated by light will flow through the upper electrode into the low resistant regions of the shunt portions, resulting in loss of electric current. Such current loss will result in decrease of open circuit voltage of the solar cell.
Since in the a-Si solar cells the sheet resistance of the semiconductor layers themselves is generally high, they need to have the upper electrode consisting of a transparent, conductive film over the entire semiconductor surface. In general, the upper electrode is a transparent, electroconductive film such as films of SnO2, In2O3, ITO (In2O3+SnO2), and so on with excellent characteristics as to transparency to the visible light and electric conductivity. These transparent, conductive films are normally formed by sputtering, vacuum resistance heating evaporation, electron beam evaporation, spraying, and so on. When there exist defects in the semiconductor layers, a considerably large amount of electric current flows into the fine defects. If the defects are located apart from a grid electrode provided on the transparent, conductive film, the resistance is high against the flow of current into the defective portions and the power loss is thus relatively small. Conversely, if the defective portions are located below the grid electrode, the defects cause greater loss of electric current.
On the other hand, in addition to the leakage of charge generated in the semiconductor layers, into the defective portions, the defective portions under existence of water produce ionic substances because of interaction with water. They gradually decrease the electric resistance at the defective portions with a lapse of operating time during use of the solar cells, whereby there appears the phenomenon of degrading the characteristics of the photoelectric conversion efficiency and others.
Incidentally, when there occur shunts as described above, the current loss can be reduced by removing the upper electrode of the transparent, conductive film at positions of the shunts. As methods of selectively removing the upper electrode at the shunt portions, there are removing techniques of immersing the solar cell in an acid, salt, or alkali electrolyte and applying a bias to the solar cell to etch the shunt portions, as disclosed in U.S. Pat. Nos. 4,451,970 and 4,464,832.
However, while these techniques describe the bias applying method and the bias applying time according to the film thickness of the transparent, conductive film, if there exist many defective portions the upper electrode will become thin throughout the entire surface to cause decrease of the photoelectric conversion efficiency, deterioration of appearance, or the like.
Of the characteristics of the solar cells, the sheet resistance of the transparent, conductive film deposited on the semiconductor layer is considered to be preferably as low as possible. Decrease in the sheet resistance of the transparent, conductive film can decrease the series resistance of the solar cells and increase the fill factor in the current-voltage curve (I-V curve) of the solar cells. Further, the decrease in the sheet resistance of the transparent, conductive film can increase the current collection efficiency and if the grid electrode is formed by attachment of wires the number of grids can be decreased and loss of light due to shadows of wires (shadow loss) can be decreased.
With the decrease in the sheet resistance of the transparent, conductive film, however, if the transparent, conductive film is removed at the shunt portions by electrochemical reaction in the electrolyte as disclosed in U.S. Pat. Nos. 4,451,970 and 4,464,832, selectivity of removal will become worse when applying the bias to the solar cell. Namely, the current becomes easier to flow to the transparent, conductive film not only at the shunt portions but also at shuntless portions, so that the transparent, conductive film will also be gradually etched at the shuntless portions during the removal of the transparent, conductive film for its entire film thickness at the shunt portions, posing the problem of significant deterioration of the characteristics and appearance of the solar cell.
The present invention has been accomplished in view of the above problems and an object of the invention is to provide a method of forming a transparent, conductive film with excellent characteristics and high yield by sputtering, without causing the shunts or the deterioration of appearance, a defective region compensation method of a semiconductor layer, a photovoltaic element, and a method of producing a photovoltaic element.
In order to achieve the above object, a transparent, conductive film forming method according to the present invention is a method of forming a transparent, conductive film on a semiconductor layer formed on a substrate, by sputtering, comprising applying voltages independently of each other to both a target and the substrate, respectively, and controlling a bias voltage appearing in the substrate so as to form the transparent, conductive film on only a portion except for a defective region of the semiconductor layer.
Preferably, at least one of the voltages applied to the target and the substrate is controlled such that a self bias of the substrate is xe2x88x9220 V to 0 V in a state in which the voltage is applied to only the target and such that a self bias of the substrate is xe2x88x9290 V to xe2x88x9230 V in a state in which the voltages are applied to both the target and the substrate, respectively, independently of each other.
Preferably, an introducing amount of a reactive gas is controlled such that a ratio of emission intensity of In during formation of the transparent, conductive film in a state in which the voltages are applied to both the target and the substrate, respectively, independently of each other to emission intensity of In during discharge in only an Ar atmosphere is within the range of 0.15 to 0.36.
A defective region compensation method of a semiconductor layer according to the present invention is a method of compensating a defective region existing in a semiconductor layer formed on a substrate, comprising:
applying voltages to both a target and the substrate, respectively, independently of each other and controlling a bias voltage appearing in the substrate to form a transparent, conductive film by sputtering; and
immersing the transparent, conductive film in an acid, salt, or alkali electrolyte and applying a bias.
A photovoltaic element producing method according to the present invention is a method of producing a photovoltaic element, comprising:
forming a semiconductor layer on a substrate; and
forming a transparent, conductive film on the semiconductor layer by the forming method as described above.
A photovoltaic element producing method according to the present invention is a method of producing a photovoltaic element, comprising:
forming a semiconductor layer on a substrate; and
compensating a defect existing in the semiconductor layer, by the compensating method as described above.
A photovoltaic element according to the present invention is a photovoltaic element comprising at least a semiconductor layer and a transparent, conductive film formed on the semiconductor layer, on a substrate, the photovoltaic element being produced by the method as described above.
A transparent, conductive film forming method according to the present invention is a method of forming a transparent, conductive film on a semiconductor layer formed on a substrate, by sputtering, comprising applying voltages independently of each other to a target and the substrate, respectively, so as to satisfy at least the following conditions of (1) and (2):
(1) a self bias of the substrate is xe2x88x9220 V to 0 V in a state in which the voltage is applied to only the target;
(2) a self bias of the substrate is xe2x88x9290 V to xe2x88x9230 V in a state in which the voltages are applied to both the target and the substrate, respectively, independently of each other.